This invention relates generally to emergency oxygen supplies for human use, and more particularly to a portable oxygen supply having a relatively small and safe reservoir of oxygen, yet making it possible for the user to breathe normally for a period equal to that obtainable from a much larger reservoir.
In terms of mechanical engineering, the human respiratory system consists of an air pump and two liquid pumps. The air pump, which is constituted by the diaphragm and the chest, acts to draw air into the lungs and then force it out again. The two ventricles of the heart function as liquid pumps, the right ventricle pumping its blood into the capillary network of the lungs to bring oxygen-short blood into the proximity of air possessing oxygen. The left ventricle pumps its blood, now rich in oxygen, into the capillary network of the body and thereby conveys rich blood into the tissues in need of oxygen.
Again in terms of mechanical engineering, the lungs may be regarded as a brilliant execution of a poor design, for lungs are inefficient organs, with the exit and entrance for air thereto at one and the same place. There is only a partial interchange of gas with each breathing cycle, and about four-fifths of the air present in the lungs is still there when the next breath begins.
Virtually, no carbon dioxide is present in the normal atmosphere, whereas expired air has about 4% carbon dioxide, and breathing becomes difficult when this percentage in the incoming air rises above 4%. If, therefore, air having the usual 20% of oxygen becomes excessively rich in carbon dioxide, a feeling of faintness is experienced. On the other hand, while oxygen is vital to breathing, one may become unduly stimulated from breathing pure oxygen for an excessive period or suffer lung damage.
Holding the breath is possible for a very limited time, say, about 45 seconds, and thereafter becomes impossible; but should one first take a few puffs of pure oxygen or gasp deeply with ordinary air, the breath can be held for perhaps 2 minutes. All respiratory system factors with respect to the purity of oxygen and the tolerable proportion of carbon dioxide must be taken into account in an emergency air supply to replace or supplement the available air in a given situation.
It is known to provide an emergency oxygen supply in the form of a valve-controlled tank of highly pressurized oxygen whose output is fed to a face mask or inhalator. With a supply of this type, the user wastes most of the available oxygen, and in order to supply sufficient oxygen for a prescribed period, one must provide a pressurized oxygen reservoir requiring a strongly-reinforced tank or canister capable of withstanding high pressures. As a consequence, the tank and the associated high-pressure control are necessarily in heavy-duty form and relatively cumbersome.
Portable air supplies designed for escape from smoke and fume-filled rooms are described in the February, l975 issue of "Compressed Air Magazine." These supplies involve canisters formed of double-coil steel tubing built to hold pressures in excess of 5000 psig and operating in conjunction with a transparent hood which covers the head of the user to keep toxic fumes out of the eyes and nose. The hood includes an exhalation valve that prevents pressure and CO.sub.2 buildup.
Also described in the same article is a Survival Support Device in which an air capsule containing air under 6500 psig provides 8 minutes of breathing air. A safety valve is provided should the pressure buildup to dangerous levels.
An individual suffering from emphysema and in need of an emergency oxygen supply is usually not in good physical condition, and the conventional air or oxygen emergency supply, though portable, nevertheless imposes a taxing burden on this purpose. Moreover, a highly-pressurized tank of oxygen in some situations, as in a fire, is potentially dangerous.
The reason why a conventional oxygen supply is wasteful is that it is designed to feed oxygen to a user in an arrangement which inherently limits its utilization to a single breathing cycle. Though a user in the course of a normal single breathing cycle inhales and exhales a large volume of air, only a small portion of the oxygen contained therein is actually interchanged in the respiratory system, while a far greater portion is taken in and then discharged without any such interchange.
Thus while a normal day's breathing involves about 530 cubic feet of air or a huge volume of air occupying a space of about 8 feet by 8 feet by 8 feet, a person confined within an unventilated chamber having this volume of air would have a sufficient air supply for much more than a day. Or to give an actual example of the ability of an individual to breathe comfortably in a confined space, we shall consider telephone booths in England which are notorious for their tight-fitting doors and lack of effective ventilation. While a telephone booth of this type holds about 37 cubic feet of air, a fair amount of which is displaced by the person making a call, it is still possible for the caller to stay in the booth and breathe with reasonable comfort for at least half an hour.
The reason for this is that the caller, though he is gradually depleting the oxygen in the air and raising the amount of carbon dioxide therein during each breathing cycle in which he takes in a relatively large volume of air, he is doing so only with respect to a small percentage of the oxygen contained in this volume. Consequently, air in the booth may be repeatedly recycled by the respiratory system before it is rendered unacceptable for breathing.
This is not to suggest that no provision be made for ventilating booths or other confined quarters, but only serves to call attention to the remarkable breathing tolerances of the respiratory system, a factor which has not heretofore been exploited in emergency oxygen supplies.